"Council is working with Queensland Rail to review stormwater drainage and hydrology plans for the Fortitude Valley catchment area," a council spokesman said.

"Some of the inner-city drains were first constructed in the 1870s."

Overall, Brisbane has an extensive drainage network, with more than 2640 kilometres of enclosed stormwater drains.

Lobby group Rail Back on Track said it believed the floodwaters that have begun to block lines could be coming from a blocked old drain.

Spokesman Robert Dow said the organisation's network of drivers and QR staff believed old drains - or the increase in developments in Fortitude Valley - were having an impact.

"What we think is that the flows around the general precinct have been altered," Mr Dow said.

"They may have been altered through development, that is a possibility.

"I've also seen something that some of the old drains could be blocked.

"Not necessarily on railway property, but adjacent which is adjusting the rainfall flow."

Mr Dow said he was glad some of Brisbane's old drains were being investigated.

"Some of the drains around that area are around 100 years old," he said.

"So it is possible that there has been some blockage in some of those old drains that hasn't been picked up, which is re-directing flows into new areas, novel areas like the Fortitude Valley Station 'swimming pool'."

This financial year council is investing $59 million to fund 70 vital stormwater drainage projects.

An extra $20 million has been allocated to repair and maintain the network.

Mr Dow said rail drivers were pleased the issue was being investigated, because it was affecting the central core of Brisbane's rail network when rain was not overwhelming.

He said while the November hail storm was significant, train drivers had reported earlier instances.

"If you have a problem in that core section, you have a problem with the whole network and it can really be disruptive to many thousands of people, particularly at peak hour," he said.